Three-dimensional measurements have been done for several years in marine seismology and offer the advantage of high information density and therefore increased information, of the availability of genuine vertical seismograms and of the possibility of making horizontal sections. Recording a three-dimensional volume of data in marine seismology is essentially accomplished using streamers towed behind ships. Frequently such streamers are towed parallel to each other.
To survey a predetermined region, the towing ship, which simultaneously tows the transmitter or sound sources, is made to move preferably in parallel passes over the total area. Thereupon a data-volume can be constructed from the surveyed tracks and, after the dynamic and static corrections and stacking have been performed, further regional profiles may be ascertained in any direction and be evaluated.
Recording a three-dimensional data-volume requires that the basic data be obtained in a tight grid. In turn, this requires that both the ship and the streamers be positioned accurately, i.e. that there position be known precisely. As a rule the tracks are surveyed while moving over a path of several kilometers, the spacing from the previous path being relatively small. In the presence of ocean currents, the streamer drift may be substantial. Even though this drift can be corrected for and eliminated when the final data-volume is constructed, it may nevertheless lead to problems when a surveying trip takes place in areas in which stationary objects such as oil platforms, buoys, shallows or other obstructions exist. Such objects on occasion may entirely prevent surveying the region by means of towed streamers. Especially where drilling platforms or other drilling structures are present, safety distances must be kept, so that measurements by streamers towed by ships are nearly impossible.